A radiation computed tomography (CT) scanner (CT scanner) used as an industrial CT scanner such as a non destructive inspection apparatus mainly includes the following three components. That is, such a CT scanner includes a radiation source configured to emit radiation, a table on which a subject is placed, the table being rotatable about a rotation center axis, and a radiation detector facing the radiation source with the table interposed between the radiation detector and the radiation source.
Any subject is placed on the table, and the table is rotated about the rotation center axis while causing the radiation source to emit radiation, thereby performing tomographic imaging. The radiation transmitted through the subject is detected by the radiation detector, and then reconstruction processing is performed on a plurality of projection images captured by the radiation detector to obtain a tomographic image of the subject. Here, on the basis of the distance from the radiation source to the rotation center axis of the table (SRD: Source-to-Rotation center Distance) and the distance from the radiation source to the radiation detector (SDD: Source-to-Detector Distance), an imaging magnification of the projection images can be defined as SDD/SRD. Therefore, accurately obtaining the imaging magnification allows dimensions of the tomographic image of the subject to be accurately grasped.
In practice, a focal position of the radiation source varies due to thermal expansion of a target that generates radiation, and properties of the radiation source such as a focal diameter also vary with a radiation condition. This causes the SDD and the SRD to vary. Therefore, unless the SDD and the SRD are accurately obtained, it is impossible to accurately grasp the dimensions of the tomographic image of the subject. In other words, when the SDD and the SRD are not accurate values, the dimensions of the obtained tomographic image of the subject deviate from a true value.
To cope with this, there are techniques of correctly obtaining the SDD and the SRD to calibrate the imaging magnification (e.g., refer to Patent Literature 1 and Patent Literature 2). As shown in FIG. 7, a linear drive mechanism MC that linearly moves a table T along an axis (also referred to as an “emitting axis”) connecting a radiation source S and a center of a radiation detector D is provided, and an appropriate calibration instrument (also referred to as a “dedicated instrument” or simply referred to as an “instrument”) whose relative position with respect to a rotation center axis is known is placed on the table T. In a technique disclosed in Patent Literature 1: JP 4396796 B2, after an image of the instrument is captured at a certain point, the instrument is linearly moved to a different point by the linear drive mechanism MC together with the table T along the emitting axis, and then another image of the instrument is captured. As described above, the image of the instrument is captured at the two points, allowing the SDD and the SRD at a specific position to be calibrated by a geometric operation. Note that in such a conventional calibration method, an instrument whose design dimension is known is used.
Further, in a technique disclosed in Patent Literature 2: JP 2013-217773 A, a shielding member having a surface inclined toward a specific focal position is disposed close to the radiation detector. When the specific focal position varies in an emitting axis direction in FIG. 11 of Patent Literature 2 (a Z direction in FIG. 11) or in a horizontal direction in FIG. 13 of Patent Literature 2 (a Y direction in FIG. 13) orthogonal to the emitting axis, a thickness of a shadow formed by the shielding member varies as shown in FIGS. 12 and 14 of Patent Literature 2. It is possible to obtain, from this shadow, a varying focal position or a reduction rate of a projection image.